Methods for coupling a fitting to a tube using a collar

ABSTRACT

A method for coupling a fitting to a tubing end includes pressing a collar over the outer surface of the tubing until the strain on the collar in the hoop direction is at a predetermined level, or pressing a collar with a predetermined axial load.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.15/616,406, filed Jun. 7, 2017, to be issued as U.S. Pat. No. 11,041,585on Jun. 22, 2021, which claims the benefit of and priority on U.S.Provisional Application No. 62/347,513, filed on Jun. 8, 2016, thecontents of both of which are fully incorporated herein by reference.

BACKGROUND OF THE INVENTION

Fittings are mounted to a tube end by being inserted into the tube endand attached to an inner surface of the tube. A collar is typicallyplaced over the tube end to “clamp” the tube to the fitting. The holdingstrength between the tube and the fitting is often inconsistent leadingto permanent failures. Thus, collars able to provide for more consistentholding strength between the tubing and the fitting so that the tubingand fitting interface can withstand loads more consistently are desired.

SUMMARY

In an example embodiment, a tube assembly includes a fitting having afirst end opposite a second end, at least a section of the fittinghaving an annular outer tapered surface reducing in diameter in adirection axially toward the first end, a tube having a portion over theannular outer tapered surface, the portion of the tube having an outersurface reducing in diameter in a direction axially toward the firstend, and an annular collar having an inner surface over the portion ofthe tube outer surface, wherein the collar in cross-section as viewedalong a plane extending axially along a diameter of the collar includesa generally triangular section and an outer section over the generallytriangular section. The triangular section defines a tapered innersurface of the collar reducing in diameter axially in a direction towardthe first end. The collar includes a fiber reinforced composite materialincluding fibers and resin. Substantially all fibers forming thetriangular section are axially oriented. The tapered inner surface ofthe collar is linear or non-linear. In another example embodiment, theouter section includes fibers oriented generally transverse to the axialdirection. In yet another example embodiment, the outer section includesfibers oriented generally perpendicular to the axial direction. In afurther example embodiment, the outer section is a generally rectangularsection when viewed along the plane. In one example embodiment, thecollar includes a first end axially opposite a second end. The generallytriangular and outer sections extend to the second end and only theouter section extends to the first end and defines a portion of theinner surface of the collar. In further example embodiment, the collarfurther includes a third section radially between the generallytriangular and the generally rectangular section. In yet a furtherexample embodiment, the collar inner surface includes two taperedsurface portions tapered at different angles in cross-section asmeasured from the plane. A first of the two tapered surface portions isthe tapered inner surface portion and the second of the two taperedsurface portions is defined on the third section. In one exampleembodiment, the portion of the tube outer surface reducing in diameteris a first outer surface section, and the tube includes a second outersurface section reducing in diameter in a direction axially toward thefirst end. The first outer surface section is between the second outersurface section and the first end, and the second outer surface sectionreduces in diameter at a steeper angle than the first section. At leastsubstantially all of the fibers in the collar third section aresubstantially axially oriented. In another example embodiment, the tubefirst outer surface section interfaces with the collar first taperedsurface portion and the tube second outer surface section is spacedapart from the collar second tapered surface portion. In yet anotherexample embodiment, the tube first outer surface section interfaces withthe collar first tapered surface portion and the tube second outersurface section interfaces with the second tapered surface portion. In afurther example embodiment, a portion of the collar outer sectioninterfaces with an outer surface of the tube. In yet a further exampleembodiment, the fitting includes a radially extending flange and thegenerally triangular section of the collar interfaces with the flangeand the outer surface of the tube reducing in diameter. In an exampleembodiment, the filler section is formed separately from the outersection and the two sections are adhered together after they are formedto form the collar.

In an example embodiment, an annular collar for applying a radialcompressive force to an outer surface of a tube includes a first endaxially opposite a second end, and. The annular collar further includesin cross-section as viewed along a plane extending axially along adiameter of the collar, a generally triangular section and an outersection over the generally triangular section. The generally triangularsection defines a tapered inner surface of the collar reducing indiameter axially in a direction toward the first end. The collarincludes a fiber reinforced composite including fibers and resin, andsubstantially all fibers forming the triangular section are axiallyoriented. The tapered inner surface of the collar may be linear ornon-linear. In another example embodiment, the outer section includesfibers oriented generally transverse to the axial direction. In yetanother example embodiment, the outer section includes fibers orientedgenerally perpendicular to the axial direction. In a further exampleembodiment, the outer section is a generally rectangular section whenviewed along the plane. In yet a further example embodiment, thegenerally triangular and outer sections extend to the second end, andonly the outer section extends to the first end. With this exampleembodiment, the outer section defines a portion of the inner surface ofthe collar. In yet a further example embodiment, the collar furtherincludes a third section radially between the generally triangular andthe generally rectangular sections. In one example embodiment, thecollar tapered inner surface includes two tapered surface portionstapering at different angles in cross-section as measured along anaxially extending plane. A first of the two tapered surface portions isthe tapered inner surface portion and a second of the two taperedsurface portion is defined on the third section. In an exampleembodiment, the filler section is formed separately from the outersection and the two sections are adhered together after they are formedto form the collar.

In an example embodiment, an annular collar for applying a radialcompressive force to an outer surface of a tube includes a first endaxially opposite a second end, and an inner surface. The collar incross-section as viewed along a plane extending axially along a diameterof the collar includes a generally triangular section and an outersection over the generally triangular section. The generally triangularsection defines a tapered inner surface of the collar reducing indiameter axially in a direction toward the first end. The collarincludes a fiber reinforced composite including fibers and resin, andsubstantially all fibers forming the triangular section are axiallyoriented. The tapered inner surface of the collar may be linear ornon-linear. In another example embodiment, the collar outer sectionincludes fibers oriented generally transverse to the axial direction. Inyet another example embodiment, the outer section includes fibersoriented generally perpendicular to the axial direction. In a furtherexample embodiment, the outer section is a generally rectangular sectionas viewed along the plane. In yet a further example embodiment, thegenerally triangular and outer sections extend to the second end andonly the outer section extends to the first end. With this exampleembodiment, the outer section defines a portion of the inner surface ofthe collar. In yet a further example embodiment, the collar furtherincludes a third section radially between the generally triangular andthe generally rectangular sections. In one example embodiment, thecollar tapered inner surface includes two tapered surface portionstapering at different angles in cross-section as measured along anaxially extending plane. A first of the two tapered surface portions isthe tapered inner surface portion and a second of the two taperedsurface portions is defined on the third section.

In an example embodiment a method for coupling a fitting to a tube endincludes inserting the fitting into the tube end such that a portion ofthe tube surrounds the fitting. The portion of the tube surrounding thefitting includes an outer surface that reduces in diameter in adirection toward the tube end. The method also includes pressing with apredetermined axial load, a collar having a tapered inner surface overthe outer surface of the tubing such that the tapered inner surfaceengages the tube outer surface. The predetermined axial load positionsthe collar at an appropriate location over the tube.

In another example embodiment a method for coupling a fitting to a tubeend includes inserting the fitting into the tube end such that a portionof the tube surrounds the fitting. The portion of the tube surroundingthe fitting includes an outer surface that reduces in diameter in adirection toward the tube end. The method also includes pressing acollar having a tapered inner surface over the outer surface of thetubing such that the tapered inner surface emerges the outer surface ofthe tubing. The collar is presssed with an axial load until the strainon the collar in the hoop direction is at a predetermine level. In yetanother example embodiment, the strain on the collar in the hoopdirection is at the predetermined level when the hoop stress on thecollar is at least 50% of the ultimate hoop stress of the collar. In afurther example embodiment, the strain on the collar in the hoopdirection is measured using a strain gauge mounted on the collar.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an example embodiment tube assemblyincluding a tube with two fittings mounted at its opposite ends of anexample embodiment.

FIG. 2 is a partial cross-sectional view of a fitting mounted into anend of the tube with an example embodiment collar over the tube.

FIGS. 3 and 4 are end views of example fittings.

FIG. 5 is an exploded view of an example fitting inserted into a tubeend.

FIG. 6A is a partial cross-sectional view of a fitting inserted into atube end and an example embodiment collar mounted over the tube end.

FIG. 6B is a partial cross-sectional view of section 6B, 6B shown inFIG. 6A.

FIGS. 7A, 7B, and 8 are partial cross-sectional views of an exampleembodiment collar interfacing with example embodiment tube ends. Thefittings are omitted.

FIG. 9 is a partial cross-sectional view of the fitting and exampleembodiment collar as shown in FIG. 6B, incorporating a collar hoopsection that is formed separate from the collar filler section and whereeach of the hoop and filler section is formed from multiplesub-sections.

DESCRIPTION

In the following detailed description, certain example embodiments areshown and described, by way of illustration. As those skilled in the artwould recognize, the described example embodiments may be modified invarious ways, all without departing from the spirit or scope of thepresent invention. Accordingly, the drawings and description are to beregarded as illustrative in nature, rather than restrictive.

With reference to FIG. 1, a composite tube assembly 1, according to anembodiment of the present invention, includes a composite tube 10. Afitting (i.e., insert) 12 and an end fitting 14 are located at each endof the composite tube 10. The composite tube 10 is a hollow andsubstantially tubular structure. With reference to FIGS. 1 and 2, thecomposite tube 10 has a body 104 having a substantially uniform outerdiameter d and a substantially uniform inner diameter d′.

The composite tube 10 may be produced by winding composite fibers in aform of a filament (and/or a tape) having an epoxy resin over a tubularmandrel. Any of a number of suitable machines known to those skilled inthe art can be used for this purpose. The composite tube may be a fiberreinforced composited formed with a thermoset or a thermoplastic resin.In other example embodiments, the composite tube may be a liquid moldedtubing that may include fiber reinforcement. The composite fibers may bewound along a direction that is substantially helical with respect to alongitudinal axis of the composite tube 10. In one embodiment, thecomposite fibers are wound at a very small helical angle (or angles)with respect to the longitudinal axis. However, embodiments of thecomposite tube 10 are not limited thereto. That is, the composite tube10 may be produced by winding filaments and/or pre-impregnated compositetapes in any known manner. The composite tube may include fibersoriented in multiple directions.

To form the tube assembly, the composite tube 10 if longer than desired(e.g., for a given application), is shortened at either one or bothends. A cutter such as an abrasive cutter or any other suitableinstrument may be used for this purpose.

With reference to FIG. 2, each fitting 12 has a first end portion 122, asecond end portion 124, and a flange (or abutment) 126 located betweenthe first end portion 122 and the second end portion 124. The fitting 12further has a third portion 128 which includes a tapered outer surfacesection 1284 located between the flange 126 and the second end portion124. Example fittings are described in U.S. Patent No. 8,205,315, thecontents of which are fully incorporated herein by reference. In theexample embodiment fittings shown in FIGS. 3 and 4, the tapered outersurface section 1284 occupies almost the entire third portion 128.

The first end portion 122 includes an entry portion 1222, a cylindricalportion 1224 and an annular tapered portion 1226. The entry portion 1222in one example embodiment has a bore 123 having internal screw threads1228. The threads 1228 extend from an open end of the entry portion 1222along the length of the entry portion 1222 towards the cylindricalportion 1224. As such, the entry portion 1222 may be embodied by a jamnut having a hexagonal outer circumference and a circular, internallythreaded inner circumference.

The cylindrical portion 1224 is located between the entry portion 1222and the annular curving portion 1226. The cylindrical portion 1224 hasan inner diameter 1225 substantially equal to an inner diameter 1227 ofthe entry portion 1222. In one embodiment, at least a portion of thebore 123 of the cylindrical portion 1224 is threaded with threads 1228.In one embodiment, the bore 123 of the cylindrical portion 1224 issubstantially unthreaded.

The annular curved portion 1226 is located between the cylindricalportion 1224 and the flange 126. A first end 1231 of the annular curvedportion 1226 extends to the cylindrical portion 1224. A second end 1233of the annular curved portion 1226 is flared and extends the flange 126.The second end 1233 of the annular tapered portion 1226 has an outerdiameter 1235 larger than the outer diameter 1237 of the cylindricalportion 1224.

The flange 126 has a substantially annular shape. An outer diameter 1240of the flange 126 is larger than the outer diameter 1235 of the annularcurved portion 1226 second end. With reference to FIG. 2, the outerdiameter of the flange 126 may be substantially equal to the outerdiameter d of the body 104 of the composite tube 10. In anotherembodiment, the outer diameter of the flange 126 is greater (or less)than the outer diameter d of the body 104 of the composite tube 10.

In an example embodiment, the tapered outer surface section 1284 tapersfrom a larger diameter to a smaller diameter in an axial directiontoward the flange. In an example embodiment, the taper of the taperedouter surface section is linear. In an example embodiment, an endportion 124 of the fitting extending to the free end 127 is not taperedand is flat (i.e., has a constant outer surface diameter) as for exampleshown in FIG. 3. In another example embodiment, the end portion 124 alsohas a tapered outer surface. In one example embodiment, the taperedouter surface 1284 of the embodiment is linear and the tapered outersurface of the end section is linear and tapers at the same angle as thetapered outer surface of the third portion so as to form one continuoustapered outer surface. This end portion serves to guide the fittingwithin the tube during assembly when the fitting is being inserted intothe tube in an effort to prevent the fitting from canting relative tothe tube.

An adhesive may be applied on the tapered area of the fitting and/or tothe inner surface of the composite tube end portion which will interfacewith the fitting. The fitting is then fitted into an end 11 of thecomposite tube. In an example embodiment, the fitting is pushed in untilthe end of the composite to abut the flange 126. The end of the tube isthen heated to a temperature sufficient for becoming thermoplastic ormoldable, i.e., a temperature sufficient to soften the resin such thatit can be molded to the tapered surface. When the heated end moldedportion cools, it compresses radially against the fitting tapered outersurface and bonds to the outer surface. With this example embodiment,Applicants discovered that due to the linear tapered of the surface, theend of the tube does not have to be axially slotted to allow for thetube to engage the tapered surface. Moreover, Applicants have discoveredthat the tube shrinks well enough on the tapered surface without foldingover itself during cooling. In an example embodiment, the taperedsurface of the fitting may be grooved with parallel grooves 131 or by ahelical groove 131, as for example shown in FIG. 4. The groovesaccommodate the softened resin from the composite tube and/or anadhesive if used, thus, allowing for a stronger bond between thecomposite tube and the fitting. The spacing and the depth of the groovesare chosen to optimize the bond between the fitting and the tube byaccommodating a proper amount of resin and/or adhesive.

With reference to FIG. 5, the second end section 124 and the thirdportion 128 of the fitting 12 are inserted into an end of the compositetube 10 until the flange 126 of the fitting 12 is a fitting the end ofthe composite tube 10. In an example embodiment, prior to the insertionof the third portion 128 and the second end portion 124 into thecomposite tube 10, the corresponding end of the composite tube 10 isaxially notched at one or more portions of the tube to facilitate, aswill be described in more detail below, an engaging of the compositetube 10 with the third portion 128 of the fitting 12. A water jet or anyother suitable instrument may be used for this purpose.

In a further example embodiment, the third portion 128 of the fitting 12is coated with an adhesive prior to the insertion of the third portion128 and the second end portion 124 into the composite tube 10.Alternatively, the inner surface portion of the end portion 150 of thecomposite tube that will interface with the fitting third portion iscoated with the adhesive. In other example embodiments, both the fittingand the composite tube are coated with the adhesive.

A collar 19 which is preformed from a composite material as shown inFIG. 2 is used as a bolstering structure. In an example embodiment, aninner surface 218 of the collar may be tapered at an angle 1500,relative to a central longitudinal axis 1502 of the fittingcomplementary to a tapered angle 1504 defined by the outer surface ofthe end portion 150 of the tube relative to a central longitudinal axis1506 of the tube, which is defined when the end portion is reduced insize to engage the tapered outer surface section 1284 of the fitting.While in the example embodiment shown in FIG. 2, the inner surface taperangle 1500 of the collar is the same as the tube end portion 150 outersurface portion taper angle 1504 of the tube. In other exampleembodiments, the taper angle 1500 of the inner surface of the collar isnot the same as the taper angle 1504 of the outer surface of the portion150 of the tube. In some embodiments, the inner surface 218 of thecollar may be non-linearly tapered. In an example embodiment, the collaris made from carbon fibers and a polymer or epoxy matrix.

In one example embodiment, after the tube is mated to the fitting, thecollar is axially slid over the fitting first end portion 12 and overthe flange 128 to engage and press on the outer surface 312 of the tube10.

In an example embodiment, after the tube is sealed to the fitting, thecollar is axially slid over the fitting first end portion 12 and overthe flange 128 to mate and press on the outer surface 312 of the tube10. The axial load required to slide the collar over the fitting andtube is referred to herein as the “axial press load.” As the collar withthe tapered inner surface 218 is slid over the end of the fitting firstand portion 12 and over the flange 126, it engages the outer surface ofthe portion of the tube portion 150 fitted over the tapered surface ofthe fitting. As the collar is further slid, it further engages andprovides further radial pressure against the portion 150 of the tubefitted over the tapered outer surface of the fitting. In an exampleembodiment, the collar is slid far enough so as to not cover the edgesurface 127 of the flange 126. In another example embodiment, however asshown in FIG. 2, the collar is slid far enough so it surrounds at leasta portion of each of the flange edge surface 127 and at least a portionof the end portion 150 outer surface 312 of the tube. In an exampleembodiment, the collar surrounds axially and circumferentially theentire flange edge surface 127. The collar enhances the integrity and/orstrength of the connection between the fitting and the tube end.Furthermore, it is expected that this configuration may be strong enoughsuch that another part of the tube may fail during axial load before theconnection between the tube and the fitting.

As shown in the example embodiments shown in FIGS. 6A and 6B, the collarincludes a filler section 300 which fills in the volume created by thereduction in size of the tube end portion 150. In the shown exampleembodiment, the filler section is triangular in cross-section. Over thefiller section is a hoop section 302, which in the shown exampleembodiment, is rectangular in cross-section. In the shown exampleembodiment, the hoop section 302 has a first portion 303 extending overthe filler section and a second portion 304 extending axially beyond thefiller section and directly over the tube. In an example embodiment, thefiller section is formed from fibers substantially all or entirely allof which are 0° fibers, i.e., fibers that extend in the longitudinaldirection (i.e., parallel to the central longitudinal axis of 1506 ofthe fitting). In the hoop section, all or substantially all the fibersare oriented primarily in a 90° direction for hoop strength (i.e.,perpendicular to the longitudinal direction). In an example embodiment,more than 50% of the fibers are orientated in a 90% direction. Inanother example embodiment, more than 80% of the fibers are oriented ina 90% direction. In a further example embodiment, more than 90% of thefibers are oriented in a 90% direction.

In some embodiments, the hoop section has multiple layers of 90° fibers,as well as layers formed from varying angles of the fibers. In addition,in the interface between the filler section and the hoop section, thefibers in the hoop sections may gradually transition to zero degree, asthey approach the filler section to alleviate interlaminate stresses.When the collar is applied over the tube, the hoop section is under hoopstress and produces an even compressive force within the filler section,which is beneficial in reducing slip load. In the example embodimentwhere the portion 304 of the hoop section extends axially beyond thefiller section, such portion 304 is not subject to any significantstress. It may be used primarily for cosmetic or environmental sealing.In an example embodiment, the hoop section second portion 304 is formedto have a clearance fit over the outer diameter of the tube body. Inthis regard, the hoop section second portion 304 is not stressed.

Because all or substantially all the fibers are oriented at 0°, hoopstrength in the filler section 300 is afforded by the resin system. Thisis minimal compared to the hoop strength of the hoop section, whichincludes the 90° fibers. As a result, by keeping the interferencebetween the tapered surface 218 of the filler section 300 (e.g., thetapered inner surface of the collar) and the tapered outer surface 312of the tube relatively consistent, then the compressive load, along bothtapers is relatively consistent. If the filler section, as for example,were to have fiber directions with more significant hoop strength, asfor example fibers oriented in the 90° direction, the stress would begreater in the direction of further reduction of diameter or more hoopstrength would be present. In addition, the 0° fibers in the fillersection are more compliant during the collar application, thus reducinglocal stress concentration that can result from small local variationsin the surface of the form tube. This has the effect of protecting thehoop section where stress concentrations may lead to premature failure.

A point 308 where the filler section, hoop and section first and secondportion meet is the inflection point where the taper of the fillersection begins or where the constant diameter surface of the insertbegins to reduce in diameter. If for design purposes a variable stressis required throughout the filler section, the interference can beadjusted by varying the inner diameter (ID) of the collar inner taperedsurface 218 to achieve the desired local stress. For example if higherstress is required the ID of the collar taper may be further reduced toprovide a greater stress. By maintaining a consistent and/or predictableinterference between the collar and the tube, the stress level of thehoop section, and therefore the filler section can be fine-tuned to anoptimum desired level. This can be confirmed with strain gauges placedover or within the hoop section. Once correlation between the axialpress load on of the collar and the stress achieved is determined for aparticular collar/tube interface, then the strain gauges can be omitted.

In other example embodiments, the outer surface 312 of the tube 10 thatinterfaces with the collar reduced in thickness forming a reducedthickness section 400. The reduced thickness section may be linearly ornot linearly tapered, may include multiple sections some of which may betapered and some which are constant thickness sections. In the exampleembodiments shown in FIGS. 7A and 7B, the tube reduced thickness section400 has two axial portions: a first decreasing thickness portion 402;and a second constant thickness portion 404 extending from the firstportion. Both portions have tapered outer surfaces 406, and 408,respectively. With these example embodiments the collar may be providedwith a third section 330 radially between the filler section 300 and thehoop section. 302. In the example embodiments shown in FIGS. 7A and 7B,the collar third section is a second filler section 332 in that it alsocontains fibers and all or substantially all fibers are fibers that arelongitudinal in that they are oriented at 0°. In other example,embodiments the second section may have other fiber orientations, as forexample a mix of 0° fibers and 90° fibers. In one example embodiment thefibers in the third section closer the hoop section are 90° fibers andthe fibers in the third section closest to the filler section are 0°fibers.

In example embodiments as shown in FIGS. 7A and 7B, the collar thirdsection includes a tapered surface 334. In the shown exampleembodiments, the tapered surface 334 extends axially beyond the taperedsurface 218 of the filler section. The collar third section taperedsurface 334 may be linear or non-linear. For example the tapered surface334 may have a curvature. In one example embodiment the collar thirdsection tapered surface 334 is complementary to the tube tapered outersurface 406.

In the example embodiment shown in FIG. 7A, the collar is slid over thetube portion 150 until the collar third section tapered surface 334mates with the tube tapered outer surface 406 and the filler sectiontapered surface 218 mates with the tube tapered surface 408. The secondportion 304 of the hoop section 302 extends directly over the tube. Inanother example embodiment, the collar is slid far enough over the tubeso that a gap 338 exists between the collar third section taperedsurface 334 and the tube tapered outer surface 406, as for example shownin FIG. 7B. With this embodiment the filler section tapered surface 218mates with the tube tapered surface 408 and the second portion 304 ofthe hoop section 302 extends directly over the gap and the tube.

In another example embodiment as shown in FIG. 8, the collar taperedinner surface 218 has a variable taper where the taper smoothlytransitions from a first taper 350 to a second taper 352 in an axialdirection toward the end 11 of the tube receiving the fitting. With thisembodiment the tube reduced diameter section 150 may have complementarysurfaces 412, 414, respectively interfacing with the collar first andsecond tapers 350, 352, respectively. As can be seen in the embodimentshown in FIG. 8, the collar section 302 does not have a second portion304 extending over a portion of the tube that is not reduced indiameter.

In an example embodiment, the structural adhesive between the formedtube and the insert has high compressive properties. In one exampleembodiment, the adhesive used has a compressive strength of 7,700 psiwith a room temperature cure and 21,000 psi with an accelerated cure.This is compared to a 4,500 psi tensile lap shear strength. Applicantshave discovered that it is beneficial to aggressively preload thisinterface in compression. This can be done through increasing tensilehoop stress on the collar. By preloading the interface, the structuraladhesive is stressed in compression along the formed taper which makesit less dependent upon the relatively low lap shear strength. At lowerpreloads, Applicants discovered dramatic reductions in bond linestrength particularly in tensile loading due to the increased relianceon the adhesives relatively lower lap shear strength.

Preloading the interface in compression may be done using variousmethods. In a first example embodiment, it is accomplished throughcontrolling the collar axially displaced location only. With thisapproach, the goal is to locate the collar and leave the hoop stress asan undefined variable. This practice results in a variable limit andultimate failure load. Variations in taper dimensions make this methodless precise than the next two. In a second example embodiment,preloading is accomplished through a set axial force. This is a goodimprovement over the axial displacement method. By guaranteeing that thepress on force is the same for each collar application the resultanthoop stress can be controlled with more accuracy. Other factors such asvariable taper friction, rate of collar application and the variationbetween static and kinetic coefficients of friction still introducesignificant variables to this method. In a third example embodiment,preloading is controlled through a set hoop stress. By placing straingauges on the collar, the precise collar preload can be determined.

In the past the collar was pushed on only to a specificdisplacement/location relative to the fitting with no regard to thetensile hoop stress or the resulting compressive preload under thecollar. In an example embodiment, an axial press load is used which maybe used in conjunction with a strain gage to determine if the desiredstress level has been achieved. In an example embodiment, the exactdisplacement/location of the collar is secondary to final hoop stress ofplaced collar. During loading of the assembly, while in use, the collarwill be subject to additional stress which must be considered in thedesign.

With this example embodiment collar, Applicants have discovered theyconsistently drive the tensile failure mode of the interface between thetube and the fitting collar. The collars may designed that they providesufficient hoop load on the tube and fitting when they are inserted aspecific amount over the tube and corresponding fitting to providesufficient holding strength to the tube an fitting, as for example whenan end of the collar 320 is flush with a free end 11 of the tube.Applicants have discovered that the example embodiment collars may bepressed with a predetermined axial press load which slides themsufficiently over the tube and corresponding fitting to consistentlyprovide desired holding strength to the tube and against the fitting. Aspreviously discussed, in an example embodiment, the axial press load isdetermined by correlating the desired stress level of the collar hoopsection, which may be determined by measuring strain using straingauges, to the axial press load required to achieve such hoop sectiondesired stress load.

Strain gauges can be applied externally to the outside of the collar andremoved after collar application. They can also be embedded in thecollar layup where they become a permanent part of the structure. Strainin the 90° direction is the most critical so at least one strain gaugewill always be oriented in that direction. If trying to confirm orquantify a constant stress (or purposefully variable stress) along thecollar multiple strain gauges are placed along one side (same angularlocation) of the collar all oriented in the 90° direction. Additionalstrain gauges can be placed in a 0° orientation and/or any angle between0° and 90° for additional clarity of the stress state of the collar.

In one example embodiment strain gauges are mounted on the collar tomeasure strain in the hoop direction. An axial press load is applied toslide the collar tapered inner surface over the tapered outer surface ofthe tube. The axial press load is applied up to a level where the strainin the hoop direction as measured by the strain gauge is at an optimumlever. In an example embodiment, the hoop strain is at an optimum levelwhen the hoop stress on the collar is about 50% of the ultimate hoopstress of the collar. In other embodiment, the ultimate strain leveloccurs when at greater the hoop stress on the collar is greater than 50%of the ultimate hoop stress. Once at the optimum hoop strain, the axialpress load is removed and the collar is adhered or otherwise fixed tothe tube and fitting.

Strain gauges can also be used in conjunction with the set axial forcecollar application method. Through development testing the set hoopstress collar application method can be used while observing the axialforce and displacement rate used to apply the collar. If a correlationcan be established between the collar press on force and the resultanthoop stress, the collar can be applied in the future with the set axialforce known to provide the ideal hoop stress.

The term “generally triangular” as used herein refer to a triangularshape and to a triangular shape that may have small variations, as forexample one or more sides of the triangle may be non-linear, e.g.curving. Similarly, the term “generally rectangular” as used hereinrefer to a rectangular or square shape and to a rectangular or squareshape that may have small variations, as for example one side or moresides of the rectangle or square may be non-linear, e.g. curving.

Although the present invention has been described in certain specificembodiments, many additional modifications and variations would beapparent to those skilled in the art. It is therefore to be understoodthat this invention may be practiced otherwise than as specificallydescribed. Thus, the present embodiments of the invention should beconsidered in all respects as illustrative and not restrictive, thescope of the invention to be determined by the claims supported by thisapplication and their equivalents rather than the foregoing description.For example, in other example embodiments, as for example shown in FIG.9, the filler section 300 is formed separately from the hoop section 302and the hoop section is bonded over the filler section. In other exampleembodiments also as shown FIG. 9, the hoop section may be formed fromseparate tubular sub-sections 1302, one over the other, and the fillersection may formed from separate tubular sub-sections 1300, one over theother. Each tubular sub-section of the hoop section and the fillersection and the hoop section and filler section may be bonded with eachother using an adhesive. In an example embodiment, adhesives are usedthat have lubricant qualities before curing that would allow onesub-section (and/or section) to glide over one another. An exampleembodiment adhesive is Hysol X adhesive. Also the sections andsub-sections may be fiber reinforced composites including a thermoset ora thermoplastic resin or may be liquid molded and fiber reinforced.Applicants have discovered by forming the filler and the hoop sectionsseparately and then attaching the hoop section over the filler section,cracks that may initiate at the filler section do not propagate into thehoop section. Similarly, cracks that may initiate in the hoop section donot propagate into the filler section. The interface between the fillerand hoop section act as a crack arrestor. Cracks are more likely toinitiate a the filler section, especially as the filler sectionthickness increases.

What is claimed is:
 1. A method for coupling a fitting to a tube endcomprising: inserting the fitting to the tube end wherein a portion ofthe tube surrounding the fitting comprises an outer surface that reducesin diameter in a direction toward the end; and pressing a collar oversaid outer surface of said tubing with a predetermined axial loadcausing a tapered inner surface of said collar to engage said tube outersurface, wherein said predetermined axial load positions the collar atan appropriate location over the tube.
 2. A method for coupling afitting to a tube end comprising: inserting the fitting to the tube endwherein a portion of the tube surrounding the fitting comprises an outersurface that reduces in diameter in a direction toward the end, andwherein the collar comprises a tapered inner surface; and pressing acollar over said outer surface of said tubing with an axial load causingthe tapered inner surface to engage the tube outer surface until thestrain on the collar in the hoop direction is at a predetermine level.3. The method of claim 2, wherein the strain on the collar in the hoopdirection is at the predetermined level when the hoop stress on thecollar is at least 50% of the ultimate hoop stress of the collar.
 4. Themethod of claim 2, wherein the strain on the collar in the hoopdirection is measured using a strain gauge mounted on the collar.